Require access to private keys for wallets

[crwatkins]

This PR codifies what I believe to be our de facto criteria for the last two years. There has been a fair amount of discussion et al. with very little support for listing wallets that do not allow access to private keys. For over two years we have not listed any new wallets that do not allow access to private keys. Instead, we have listed such submissions elsewhere on the site under other categories.

[schildbach]

Just checking, this would essentially remove Coinapult, Xapo and Coinbase from the web wallets section? All others grant you full or shared control over your money.

[crwatkins]

@schildbach I left that out of this PR for now so that we could discuss the criteria and let it stand on its own, but after we all settle on any new criteria I would submit a new PR to update the listings. If the wording remains the same, the answer to your question is yes those web wallets would most likely be removed. Note that in this PR we already moved these wallets, but have not yet removed them from the wallet listings.

Note that Coinbase has a non-default wallet mode which I believe allows for access to private keys. Even though our normal criteria are applied against default operating modes, I would be willing to consider this for an exception if Coinbase or another party would be interested in proposing and supporting the listing.

[wbnns]

Unless others object, this will be merged on Wednesday, August 2nd.